Projection operation in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Projection in Relational Algebra

Which of the following best describes the projection operation in relational algebra?

A It selects specific columns from a table, removing duplicates by default.

B It selects specific rows from a table based on a condition.

C It combines two tables by matching rows with common attributes.

D It deletes rows from a table permanently.

💡 Hint

Think about which part of the table (rows or columns) projection focuses on.

Projection in relational algebra extracts specific columns (attributes) from a relation (table). It also removes duplicate rows in the result.

📋 Factual

intermediate

Result of Projection Operation

Given a table Students(Name, Age, Grade), what will be the result of the projection operation π_Name, Grade(Students)?

A A table with only the first row's Name and Grade.

B A table with columns Name, Age, and Grade, including all rows.

C A table with columns Age and Grade, including all rows.

D A table with columns Name and Grade, including all rows, duplicates removed.

💡 Hint

Projection keeps only the specified columns and removes duplicates.

The projection π_Name, Grade returns only the Name and Grade columns from the Students table, removing any duplicate rows.

🔍 Analysis

advanced

Effect of Projection on Duplicate Rows

Consider a table Employees(ID, Department, Salary) with multiple employees having the same Department and Salary. What happens when you perform the projection π_Department, Salary(Employees)?

A The result includes only the Department column with duplicates removed.

B The result includes unique combinations of Department and Salary, duplicates removed.

C The result includes all columns with duplicates removed.

D The result includes all rows with Department and Salary, duplicates kept.

💡 Hint

Remember that projection removes duplicate rows after selecting columns.

Projection selects the specified columns and removes duplicate rows, so only unique Department and Salary pairs remain.

❓ Comparison

advanced

Projection vs Selection

Which statement correctly compares projection and selection operations in relational algebra?

A Both projection and selection choose rows but differ in filtering criteria.

B Projection chooses rows; selection chooses columns based on conditions.

C Projection chooses columns; selection chooses rows based on conditions.

D Both projection and selection choose columns but differ in filtering criteria.

💡 Hint

Think about whether each operation filters rows or columns.

Projection extracts specific columns, while selection filters rows based on conditions.

❓ Reasoning

expert

Projection Operation Output Size

Given a relation R(A, B, C) with 100 rows, where column A has 50 unique values, column B has 80 unique values, and column C has 100 unique values, what is the maximum number of rows in the result of π_A, B(R)?

A 100

B 80

C 50

D 130

💡 Hint

Consider that projection removes duplicates but the combination of A and B may be unique for each row.

The maximum number of rows after projecting columns A and B is at most the total number of rows in R, which is 100. It cannot exceed the original row count.

Practice

1. What does the projection operation in a database primarily do?

easy

A. Select specific columns from a table

B. Combine rows from two tables

C. Delete rows from a table

D. Sort rows in ascending order

Solution

1. Step 1: Understand the purpose of projection

   Projection is used to choose certain columns from a table, not rows or sorting.

2. Step 2: Compare options with definition

   Only Select specific columns from a table matches the definition of projection as selecting columns.

3. Final Answer:

   Select specific columns from a table -> Option A

4. Quick Check:

   Projection = Selecting columns [OK]

Hint: Projection means picking columns only, not rows or sorting [OK]

Common Mistakes:

• Confusing projection with selection (which filters rows)
• Thinking projection changes row order
• Mixing projection with join operations

2. Which of the following is the correct SQL syntax for projection to get columns name and age from a table students?

easy

A. SELECT * FROM students WHERE name, age;

B. SELECT name age FROM students;

C. SELECT name AND age FROM students;

D. SELECT name, age FROM students;

Solution

1. Step 1: Recall correct SELECT syntax

   The correct syntax uses SELECT followed by column names separated by commas, then FROM table.

2. Step 2: Check each option

   SELECT name, age FROM students; matches the correct syntax: SELECT name, age FROM students;

3. Final Answer:

   SELECT name, age FROM students; -> Option D

4. Quick Check:

   Correct SELECT syntax = SELECT name, age FROM students; [OK]

Hint: Use commas between columns after SELECT, then FROM table [OK]

Common Mistakes:

• Using * instead of column names for projection
• Using AND instead of comma between columns
• Omitting commas between column names

3. Given a table employees with columns id, name, and salary, what will the result of SELECT name FROM employees; be?

medium

A. All rows with only the name column values

B. All rows with id and name columns

C. All rows with all columns

D. Only the first row with name column

Solution

1. Step 1: Understand the SELECT statement

   SELECT name FROM employees returns all rows but only the 'name' column values.

2. Step 2: Compare options with expected output

   All rows with only the name column values correctly describes the output as all rows with only the 'name' column.

3. Final Answer:

   All rows with only the name column values -> Option A

4. Quick Check:

   Projection selects columns, so only 'name' column shown [OK]

Hint: SELECT column returns all rows but only that column [OK]

Common Mistakes:

• Thinking it returns only one row
• Expecting multiple columns without listing them
• Confusing projection with filtering rows

4. Identify the error in this SQL query for projection: SELECT name age FROM employees;

medium

A. SELECT keyword is misspelled

B. Table name is incorrect

C. Missing comma between column names

D. No error, query is correct

Solution

1. Step 1: Analyze the column list syntax

   Column names must be separated by commas in SELECT statements.

2. Step 2: Check the given query

   The query has 'name age' without a comma, which is a syntax error.

3. Final Answer:

   Missing comma between column names -> Option C

4. Quick Check:

   Columns need commas between them [OK]

Hint: Always separate columns with commas in SELECT [OK]

Common Mistakes:

• Forgetting commas between columns
• Assuming space works as separator
• Ignoring syntax errors in column list

5. You have a table orders with columns order_id, customer_id, product, and quantity. You want to create a new table order_summary that only stores order_id and quantity. Which SQL statement correctly uses projection to do this?

hard

A. CREATE TABLE order_summary SELECT * FROM orders;

B. CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders;

C. INSERT INTO order_summary SELECT customer_id, product FROM orders;

D. CREATE order_summary AS SELECT order_id, quantity FROM orders;

Solution

1. Step 1: Understand creating a new table with selected columns

   To create a new table with specific columns, use CREATE TABLE ... AS SELECT with projection.

2. Step 2: Evaluate each option

   CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; correctly creates the table with only order_id and quantity columns from orders.

3. Final Answer:

   CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; -> Option B

4. Quick Check:

   Projection + CREATE TABLE AS SELECT = CREATE TABLE order_summary AS SELECT order_id, quantity FROM orders; [OK]

Hint: Use CREATE TABLE ... AS SELECT columns for projection [OK]

Common Mistakes:

• Using SELECT * instead of specific columns
• Omitting CREATE TABLE keyword
• Selecting wrong columns for the new table